The Role of Chirality-Induced Spin Selectivity in Helicene-Based Photogenerated Radical Pairs

Chirality-induced spin selectivity (CISS) revealed a close connection between molecular chirality and electron spin. Because CISS is observed even at room temperature, it offers a promising route toward spin-based technologies operable under ambient conditions. However, its microscopic origin remains the subject of debate. In recent theories, the parameters governing the electron motion through the chiral bridge play a key role in CISS efficiency. To disentangle this specific contribution from that arising from the overall intrinsic chirality of the molecule, we synthesized a new chiral donor–acceptor dyad (Dχ–B–A) incorporating a thia-bridged[4]helicene donor, known to have high CISS efficiency in transport experiments, and a perylene diimide (PDI) acceptor connected by a three-ethynylbenzene bridge. Transient absorption measurements at 85 K show that photoexcitation of PDI generates a long-lived radical pair (Dχ·+–B–A·–) with a lifetime exceeding 500 ns. The combined analysis of the spin polarization mechanism using time-resolved electron paramagnetic resonance, DFT calculations, and theoretical modeling indicates weak CISS polarization and suggests that CISS efficiency is higher in the presence of a chiral bridge.